The invention relates to a circuit for demodulating at least one modulated signal, preferably a measuring signal of a sensor, the circuit comprising at least one input, with the signal being applied to the input. The invention further relates to a method of demodulating a modulated signal, preferably a measuring signal of a sensor, in particular for operating a demodulation circuit having at least one input, with the signal being applied to the input.
Circuits for demodulating signals have been known in practical operation for a long time. The signal could be, for example, a measuring signal of a sensor system, which is used to measure distances in a noncontacting manner, as disclosed, for example, in DE 42 25 968 A1 and corresponding U.S. Pat. No. 5,629,619. In this case, the sensor system is an inductive displacement sensor, which is operated at relatively low frequencies in a range from 1 kHz to 10 kHz. The known sensor system comprises a sensor coil that is activated in phase opposition, with a ring overlying it in noncontacting relationship. This ring influences the inductance of the sensor coil corresponding to the position of a test object. In the center of the sensor coil, a plurality of taps are located, which are averaged and amplified with a difference selection amplifier. Since the sensor coil is operated with an ac voltage signal, an ac voltage signal arises at the output of the difference selection amplifier, and in addition a voltage, which corresponds to the actual position of the ring and the transient behavior of the real sensor coil circuit. This signal must now be first demodulated. To this end, the input signal is again multiplied by the carrier frequency. It is then filtered by means of a low-pass filter. In this instance, it is already known to modulate with a sine-wave signal or a square-wave signal, which has a favorable effect in the processing of signals.
The known modulation is problematic in particular in that the occurring time constants of the filtration are very great, and that the circuit used for this purpose is made relatively large because of its setup and, consequently, is unsuited for an application, which has available only a small space for the circuit.
It is therefore an object of the present invention to provide both a demodulation circuit and a demodulation method of the initially described type, which enable a use of the circuit even when the available space for the circuit is small.